1. Field of the Invention
The present invention relates to a rotating anode X-ray tube assembly. In particular, the present invention relates to a structure for improving the heat dissipation characteristics of an anode.
2. Description of the Related Art
A conventional rotating anode X-ray tube assembly for improving the heat dissipation characteristics of an anode is largely classified into the following two types.
(1) Type 1: A rotating anode X-ray tube assembly includes a rotating anode X-ray tube and a housing, etc. The rotating anode X-ray tube is provided to receiving a rotatably supported anode target in a vacuum envelope. The housing is provided to receive a rotary anode X-ray tube. In order to remove the heat of the anode target, a circulation path for circulating a cooling medium in the anode target is provided (e.g., see Jpn. Pat. Appln. KOKOKU Publication No. H5-27205 and Jpn. Pat. Appln. KOKAI Publication No. 2006-54181).
The heat of the anode target is conducted to a cooling medium via a short thermal path. Therefore, the heat dissipation characteristics of the anode is improved.
(2) Type 2: A rotating anode X-ray tube assembly including the following components:
One is a vacuum chamber, that is, a vacuum envelope rotatable around the axis line, and given an anode target as its part. Another is means for rotating the vacuum envelope around the axis line. Another is a cathode generating electrons, attached in the vacuum envelope, and a deflection coil arranged out of the vacuum envelope to deflect the electrons into an area out of the axis line of the anode target. Another is a slip ring mechanism for supplying current to the cathode via a wall portion of the vacuum envelope from an external source of the vacuum envelope (e.g., see Japanese Patent No. 2539193, French Patent Application No. 2599555-A1, Japanese Patent No. 2929506 and U.S. Pat. No. 6,396,901).
The heat of the anode target is conducted to a cooling medium via a short thermal path. Therefore, the heat dissipation characteristics of the anode is improved.
The rotating anode X-ray tube assembly having the foregoing structure (1) has the following problem. Specifically, if the thermal load of the rotating anode X-ray tube becomes large, required cooling performance is not sufficiently obtained for the following reasons.
A) The difference (relative moving speed) between a moving speed of the backside of the rotating anode target and that of fluid contacting with the backside is high. In this case, the thermal conductivity at the contact interface increases. However, in the case of the foregoing (1) structure, the relative moving speed does not so depend on a rotating speed of the anode target, and almost depends on a fluid speed of the cooling medium only. This is because the cooling medium rotates together with a rotation of the anode target (the case of Jpn. Pat. Appln. KOKAI Publication 2006-54181).
B) The cooling medium is forcedly supplied by a circulating pump via the inside of a thin shaft having a high fluid resistance and a narrow path provided in the target. For this reason, there is a limit to improving the fluid speed of the cooling medium.
C) According to the structure in which a flow path is provided in the anode target, the manufacturing cost increases resulting from its complication. Conversely, according to the structure shown in FIG. 5 of Jpn. Pat. Appln. KOKAI Publication 2006-54181, no flow path is provided in the anode target. However, the foregoing simple anode target structure is employed, and thereby, cooling performance is further reduced.
The rotating anode X-ray tube assembly having the foregoing (2) structure has the following problem like the rotating anode X-ray tube assembly having the foregoing structure (1). Specifically, if the thermal load of the rotating anode X-ray tube becomes large, the required cooling performance is not sufficiently obtained for the following reasons.
D) First, it is difficult to use a water cooling medium having high cooling performance. Insulation oil having low cooling performance must be used as the cooling medium. In other words, a space where the cooling medium exists and a cathode potential exposed space communicate with each other. For this reason, if the water cooling medium is used, breakdown voltage of the cathode is reduced resulting from an influence of water vapor.
E) The following structure is given; specifically, there is provided a slip ring mechanism for supplying current to the cathode via a wall portion of the vacuum envelope from an external source of the vacuum envelope. Resulting from the foregoing structure, it is difficult to realize highgrade functions such as multiple focus or a pulsed operation in addition of a grid electrode. This is because many slip ring mechanisms must be provided in accordance with the highgrade functions. As a result, one or more slip ring mechanisms must be provided at a portion having high circumferential speed out of the axial line. Such a case, the lifetime of the slip ring mechanism is shortened due to abrasion of the sliding parts.